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Thread: Annealing hard metal in a fireplace?

  1. #1
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    Default Annealing hard metal in a fireplace?

    I've seen it mentioned on the forum that hardened steel can be annealed in a wood burning fireplace. After a few years of not using ours, SWMBO & I decided to fire it up this winter (friggin' cold!)

    I have a 2 1/2 ft. piece of auto axle that I drug home a couple years ago, thinking I'd make something with it. The darned thing is hard as Superman's kneecap and I'd like to soften it. If I simply laid it on the bed of coals it seems to me that it would slowly settle down through the coals and end up too close to the fireplace floor to stay hot.

    Do you just lay it across the top of the andirons, pile on the wood and forget about it for a day? Or do you tie it loosely to the bottom side of of the andirons so it'll stay close to the top of the coals? If so, what kind of wire?
    Milton

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  2. #2
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    Let the fire but down to to a good bed of red hot coals higher than your grate. Lay the steel on the bed of coals and build a good fire on top. Let that buren down to a bed of coals and rake the coals up over the steel and leave it to cool down slowly. As long as you got the steel red hot after it cools slowly it should be fully annealed. If you did not get it hot enough you could repeat. Gary P. Hansen

  3. #3
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    I welded up an annealing box from 1/8" plate, filled it with ash from the fire place and it works perfectly. Get the material red hot, put it in the box and next day take it out......files etc come out very soft.

    Now, at 2.5 feet you don't want to weld up a box, but the take away is 1) that ash provides a great insulator and isn't bother by the high temps and 2) as Gary said, it won't work if you can't get the metal hot enough....I can't see how the fireplace will get you there, especially for a 2.5' piece, then again i haven't tried so maybe its possible

    can you torch into some manageable sizes ?

    When i need to get a largish piece, like a foot long, red hot, I build a temp structure out of insulated firebrick, bottom, sides, roof and heat the item red hot there and drop it in the annealing box. Last one i did was a big ass file - with the IFB structure i was able to leave the O/A in the corner and got it done with propane, cheap sob that i am.

    anyway, your challenges are getting it to temp and then having it cool slowly.....I'd be thinking the fireplace won't generate enough heat but the ashes you make could be very handy. maybe set up on a couple of pieces of brick in the fireplace (without the grate), lay the piece across the the brick more or less surrounded by ash and get to temp with a a torch then cover it over with ash. just an idea, maybe some domestic issues with wheeling the O/A cart into the house though
    Last edited by Mcgyver; 01-21-2009 at 10:45 PM.

  4. #4
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    I don't think a fireplace will do the trick either. It has too much airflow to get hot enough. I anneal my pieces in our airtight wood stove which is a great big old Fisher with a box about three feet deep and lined with firebrick on the bottom half. I build up a good fire and let it burn down the toss in the material and keep the fire stoked as usual. Then I let it burn out and cool overnight. Next am it is still pretty hot but below the quenching temperature so it can come out and be washed.
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    I don't think you need to get it all that hot, relatively speaking. It's almost certainly just high-carbon steel. To anneal it, you don't need to get it as hot as you would if you were trying to harden it. In fact, just "dull red," then cooled slowly, ought to do it.
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  6. #6
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    Thanks all. I'll try Gary's method since it looks easy. As luck would have it, the temps are climbing and are supposed to be back in the 60's (!!) on Sat. when I was going to try. Never fails; plan something & the weather won't cooperate. I'm not going to complain too loud though.

    McG I was hoping to get the middle soft enough to cut it in half in my bandsaw and then throw 'em back in again. I don't have a torch yet but do have my eye on a set.

    As far as the heat goes, if I open my damper wide open and close the glass doors, leaving the little sliding vent at the bottom open, the coals get REAL hot. I've seen the middle bars on the grate glowing pretty red! I guess I could hit it with a hairdryer to stoke it up even more if need be.
    Milton

    "Accuracy is the sum total of your compensating mistakes."

    "The thing I hate about an argument is that it always interrupts a discussion." G. K. Chesterton

  7. #7
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    Sorry, the fireplace trick does work. Fuel and location may change but the fireplace was the standard annealing furnace for Muddle Ingineers in the UK before the "Central Heating" brigade took over.

    The fuel used was coal and the text of many articles in the ME were " Place in the burning fire and leave overnight "

    Regards Ian.

    Annealing temperature is dependent on material being annealed.
    Last edited by Circlip; 01-22-2009 at 10:10 AM.
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  8. #8
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    I think it all depends on this question --- "can you get it glowing cherry red"

    If so then your fireplace will work just fine, some things to consider

    hardwood creates hotter coals that last much longer -- something like pine will due for a piece thats not as thick but a fiery bed of hardwood coals will get some fairly large material cherry red hot.

  9. #9
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    I've annealed a car axle in my fireplace and there wasn't much to doing it. I laid the axle on the grate (about 4" above the bricks), built a fire using relatively small wood (so it would burn fast and hot) and kept it going for a couple hours. The fire was much hotter than I expected - the axle actually turned dull red.

    I left it on the grate over night to cool slowly. The axle, which had been really difficult to cut before annealing, turned very nicely afterward and became several MT3's for my mill tooling. The axle spline is visible on the leftmost holder:




    John

  10. #10
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    Quote Originally Posted by GadgetBuilder
    The fire was much hotter than I expected - the axle actually turned dull red.
    Steel's annealing temperature (when the carbides go back into solution) is 1341 F, which is dull red

    Anything below that temperature won't do much.
    "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

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